ULTRASTRUCTURE OF SEROTONIN-IMMUNOREACTIVE TERMINALS IN THE CORE AND SHELL OF THE RAT NUCLEUS-ACCUMBENS - CELLULAR SUBSTRATES FOR INTERACTIONS WITH CATECHOLAMINE AFFERENTS

The nucleus accumbens is composed of a core region involved in motor f unctions and a shell region implicated in emotional and motivational p rocesses. Both of these regions receive serotonin- and dopamine-contai ning afferents. We examined whether the serotonin innervation or relat ion to catecholamine (mainly dopamine) axons in the nucleus accumbens shows common features or specializations corresponding to the noted fu nctional differences in core and shell subregions. To address this que stion, we examined the ultrastructure of serotonin-containing axons an d their relation to catecholamine-containing afferents in either the c ore or shell of the nucleus accumbens. Single coronal sections through the rat forebrain were processed for immunoperoxidase labelling of se rotonin and immunogold silver labeling of tyrosine hydroxylase, the ca techolamine-synthesizing enzyme. Varicose processes showing peroxidase product for serotonin by light microscopy were confirmed to be axons and terminals by electron microscopy. In a quantitative analysis of se rotonin-immunoreactive terminals forming one or more contacts in singl e sections, some common features were observed. For the core (n = 120) and the shell (n = 82), 41% formed synaptic junctions with unlabeled dendrites, 75% were in apposition with unlabeled terminals, which ofte n formed asymmetric junctions, and 20% were in apposition with axons o r terminals containing tyrosine hydroxylase. Thus, in both the core an d shell of the nucleus accumbens, serotonin terminals synapse on posts ynaptic neurons and are likely to modulate or be modulated by presynap tic interactions with excitatory axons forming asymmetric junctions an d by catecholaminergic afferents. Marked differences in the morphology of serotonin axons were also seen in the core versus shell of the nuc leus accumbens. By light microscopy, serotonin-immunoreactive axons we re thicker and more varicose than those found in the core. Ultrastruct ural analysis confirmed that, in contrast to the core, serotonin-immun oreactive axons and terminals in the shell were larger in cross-sectio nal diameter size (0.7 mum vs. 0.3 mum). Additionally, serotonin axon terminals in the shell contained more numerous immunoreactive large de nse core vesicles and more frequently formed symmetric as opposed to a symmetric contacts with dendrites. The larger size and more numerous d ense core vesicles in serotonin-immunoreactive terminals in the shell support the concept that serotonin or co-existing neurotransmitter may be more tonically released in the shell versus core of the nucleus ac cumbens. (C) 1993 Wiley-Liss, Inc.